A. Field Of The Invention
The present invention generally relates to the grounding of a bushing for a stirrer shaft of a stirrer paddle in a microwave oven and, more particularly, to a new and improved method and apparatus for the grounding of the stirrer bushing by having serrations on the head of the stirrer bushing abrade against raised ridges on a wall of a cavity in the microwave oven.
B. Description Of The Prior Art
Microwave ovens are used in the home and in commercial establishments to heat food items placed within a cooking cavity of the microwave oven. In certain microwave ovens, a stirrer blade or paddle is disposed in a cavity located above the cooking cavity forming a portion of a waveguide duct extending from a magnetron producing the microwave energy to the cooking cavity. The stirrer paddle is used effectively to stir the microwave energy that is being transmitted through the waveguide duct to the cooking cavity in order to enhance the distribution of the microwave energy delivered to the cooking cavity. As a result, the food items in the cooking cavity are more uniformly heated.
One such type of microwave oven having a stirrer is disclosed in U.S. Pat. No. 4,618,756; and another is disclosed in U.S. Pat. No. 4,429,204. In both of these patents, the stirrer paddle is disclosed as being mounted in the microwave energy feedbox or waveguide duct located above the cooking cavity. A stirrer shaft for rotating the stirrer paddle extends through the top wall of the waveguide duct from a mechanism used to rotate the stirrer shaft and thereby the stirrer paddle.
In order to permit the stirrer shaft to rotate, the stirrer shaft may extend through the top wall in a stirrer bushing. The stirrer bushing has an externally threaded stem portion which may be threadedly engaged into a threaded passage of a stirrer nut affixed to the top wall by applying an appropriate torque to a head portion of the stirrer bushing disposed at one end of the stem portion. The stirrer bushing must be grounded to the top wall of the waveguide duct so as to prevent RF leakage from occurring from the waveguide duct to the outside of the microwave oven. Even though the underside surface of the head of such a stirrer bushing engages the top wall of the waveguide duct when the stirrer bushing is threaded into the stirrer nut, some prior art stirrer bushings had a tendency to be insufficiently grounded to the top wall of the waveguide duct because the top wall normally is painted.
In the above-mentioned U.S. Pat. '204 patent, a nut is disclosed as securing a bushing in a top wall of a cavity located above a cooking cavity of a microwave oven. The nut may be provided with a serrated face to improve the electrical conductivity to the cavity if the wall is painted (column 3, lines 32-35). However, such a serrated nut does not ensure that the stirrer bushing is consistently grounded to such a painted wall.
U.S. Pat. Nos. 1,783,446; 1,875,930; 3,213,914; 3,750,525; 4,543,023 and 4,557,651 disclose fasteners having some type of irregular surface, groove or recess on the head or shank of the fastener to secure the fastener to a panel or the like. However, none of these prior art patents disclose or suggest any way to enhance the grounding of a fastener to a surface or wall when that surface is painted as is the case with the painted internal wall surfaces in a microwave oven.